In recent years, along with weight saving, downsizing and high densification of electronics products, demands for printed circuit boards are increasing. Particularly, demands for flexible metal laminates (also called flexible printed circuits (FPC) or the like) are increasing. A flexible metal laminate has a structure having a circuit consisting of a metal foil formed on an insulating film.
A flexible metal laminate is commonly produced by heating and contact-bonding a metal foil to the surface of a substrate by means of an adhesive material, employing as the substrate a flexible insulating film formed of an insulating material. As the insulating film, e.g. a polyimide film is preferably used. As the adhesive material, a thermosetting adhesive such as an epoxy or acrylic adhesive is commonly used.
A thermosetting adhesive is advantageous in that adhesion at a relatively low temperature is possible. However, in future, FPCs using a thermosetting adhesive are considered to be insufficient as required properties such as heat resistance, bending property and electrical reliability become strict.
As a FPC using no thermosetting adhesive, a FPC having a metal foil directly bonded to an insulating film and a FPC using a thermoplastic polyimide for the adhesive layer have been proposed (Patent Document 1).
Further, a laminate for a flexible printed wiring having a reinforcing layer of e.g. a polyimide resin and an electrical conductor layer such as a copper foil laminated via an electrical insulator layer made of a fluorinated copolymer having acid anhydride groups has been proposed (Patent Document 2). The fluorinated copolymer has adhesion, and the electrical insulator layer functions as an adhesive layer. Further, the fluorinated copolymer is a fluorinated resin excellent in electrical properties and by using it as a layer in contact with the electrical conductor layer (metal foil), excellent electrical reliability as compared with a case of using a thermoplastic polyimide will be obtained.